Alkyl xanthogen formate mixture as flotation agent

ABSTRACT

A batch process including a circulating reactor and an agitated reactor for the production of dialkyl xanthogen formates, including means to increase yields of products, inhibiting chloroformate hydrolysis, suppress production of xanthic anhydride as a byproduct, and generally control product/byproduct yields based on intended use. Certain product mixtures are particularly useful as flotation reagents, specifically collectors in the flotation of molybdenum-bearing copper ores, where molybdenum recovery has been increased by more than 10% without adverse effects on copper recovery.

BACKGROUND OF THE INVENTION

This invention relates to an improved industrial process for themanufacture of dialkyl xanthogen formates with the general formula:##STR1## where R and R' are alkyl groups that can be identical ordifferent, straight chain or branched, and that contain from 1 to 6carbon atoms. The hydrogen groups may also be replaced by halogen atoms,cyanide groups or other nitrogen compounds. Unsaturated groups have alsobeen shown to be useful in mineral reagents, pesticides and asaccelerators.

These compounds have been described as useful collectors in a number ofexpired patents, but there are no process patents and only laboratorypreparations are described in the literature. The main patents on thesubject and the examples on synthesis of these products are U.S. Pat.Nos.:

Douglass 1,652,099 (1927),

Fischer 1,684,536 (1928),

Fischer 2,608,572 (1952) and

Twiss et al, (U.K. Pat. No.) 353,871 (1931).

Generally, these patents disclose laboratory procedures for preparationof compounds of this general type, but do not comment on yields orbyproduct formation.

The patents cited give examples of the preparation of xanthogen formatesaccording to the reaction: ##STR2## However, industrial processes havenot been published or described.

The few articles on the reactions of alkali metal alkyl xanthates withalkyl chloroformates report that there are various competitive reactionswhich result in a reaction product which can contain impurities that areinert in the flotation process. These reactions consume a considerableproportion of the expensive starting materials, such as the alkylchloroformates, increasing production costs.

The main articles published that describe these reactions are:

J. Welde, Journ.f.prakt. Chemie., (2) Band 15, p. 44 (1887)

R. Holmberg, Journ.f.prakt. Chemie., (2) Band 71, p. 264 (1905)

A. Cambron, Canadian Journal of Research, 2, p. 341 (1930)

R. Sayre, J.Am. Chem. Soc., 74, p. 3647 (1952)

R. Felumb, Bull. Soc. Chim. France, p. 890 (1957)

S. M. Gurvich et al, Zh. Prikl. Khim, 39, (2) p. 531 (1966)

From these, different reaction conditions for reaction (1) result in areaction product containing:

    ______________________________________                                        (I)  Dialkyl xanthogen formate                                                                    ##STR3##         0 to 80%                                 (II) Dialkyl xanthic anhydride                                                                    ##STR4##         10 to 100%                               (III)                                                                              Dialkoxy carbonyl sulfide                                                                    ##STR5##         0 to 40%                                 (IV) Dialkyl carbonate                                                                            ##STR6##         0 to 20%                                 Unreacted raw materials and other impurities                                                               0 to                                                                          10%                                              ______________________________________                                    

The reaction mechanisms that result in this product mix have beenstudied by Cambron, Op. cit. and S. M. Gurvich et al, Op. cit., and inwork leading to the present invention.

Welde found that the uncontrolled reaction between potassium ethylxanthate and ethyl chloroformate in water resulted in the production ofonly xanthic anhydride (II) and diethyl carbonate (IV). Holmberg usedice to cool the reaction and obtained a mixture of compounds (I) and(II), observing that a xanthate excess resulted in an increase inproduction of xanthic anhydride (II). Gurvich et al studied the reactionbetween potassium butyl xanthate and methyl chloroformate and found thatCOS was evolved and that the reaction product analysis varied as afunction of the reaction conditions, yielding:

Compound (I): 60 to 100%

Compound (II): 0 to 30%

Compound (III): 0 to 8%

To explain this composition range they postulated the followingreactions: ##STR7## They postulate that the intermediate reactionproduct of reaction (2) is unstable and loses COS to form the xanthicanhydride.

The mechanism proposed by Gurvich implies that the xanthate anionattacks the C═O group of the xanthogen-formate, but this group is lessactive than the C═S group. Further, reaction (4) is unlikely, as if itis done independently in the laboratory yields do not reach 30%. Basedon this and the work reported by Cambron and current work, the moreprobable reaction mechanism is: ##STR8##

The production of compound (IV), COS and other impurities can beaccounted for, in the presence of water, which is normally added to thereaction or produced in the systhesis of the xanthate, by the followingreactions: ##STR9##

Several workers have found that high reaction temperatures favoured theproduction of compound (IV), which is not surprising as reaction (7) isthe recommended synthesis for diethyl carbonate when carried out at 80°C. Holmberg's observation that adding the chloroformate to the xanthatefavoured the production of xanthic anhydride can be deduced fromreactions (2) and (5).

Cambron (p. 346) describes an experiment which supports reaction (2)when xanthate is added to a xanthogen formate.

OBJECTS OF THE INVENTION

The objects of this invention are an improved industrial process for themanufacture of dialkyl xanthogen formates based on the reaction of anaquous solution of an alkali metal alkyl xanthate with an alkylchloroformate which:

Increases the yield of the final product;

Provides a method of controlling side reactions to give a predictablefinal composition and thus optimize the metallurgical and physicalproperties of the froth flotation collectors produced;

Provides a method of controlling the freezing point of the collectorindependently of the collector composition; and

Provides an equipment design which is capable of producing dialkylxanthogen formates in which either alkyl group may have from 1 to 6carbon atoms, the alkyl groups can be branched or straight chained,saturated or unsaturated, and where hydrogens can be substituted withmore polar groups such as halogens, cyanides or other nitrogen groups.

Various other objects and advantages of the invention will become clearfrom the following description of embodiments, and the novel featureswill be particularly pointed out in connection with the appended claims.

THE DRAWING

Reference will hereinafter be made to the accompanying drawing, which isa simplified schematic flow sheet or flow diagram illustrating anembodiment of the invention.

DESCRIPTION OF EMBODIMENTS

The essential element of maintaining control over the reaction between achloroformate and a xanthate is to provide means of restricting thereaction to a zone where the relative proportions of the two reactingphases is fixed and relatively constant, and where there is good controlover the reaction temperature. A continuous process is economicallyun-attractive because the alkyl chloroformate is very soluble in thexanthogen formate, as well as being highly corrosive, so that if thereaction is carried out with a considerable excess of chloroformate,which minimizes xanthic anhydride production, the end product would haveto be distilled to remove unreacted chloroformate. As the xanthogenformate is thermally unstable, this distillation necessarily would haveto be a vacuum operation. A batch reaction has a continuosly variableratio of xanthate to chloroformate, which means that the only way tocontrol the xanthic anhydride content in the final product is bymaintaining a very low reaction temperature, which reduces the reactionrate and requires long cycles and large reactors with large energyconsumption in agitation. The process of the invention simulates acontinuous reactor by employing a pump/heat exchanger combinationrecirculating the different phases from a storage reactor and combiningthem with fresh xanthate solution, which then reacts in a heat exchangerat a controlled temperature. The reaction is then finished by startingup an agitator in the storage reactor. The flow sheet is shown in thedrawing.

The process consists of a unit where an aqueous or alcoholic solution ofalkali metal alkyl xanthate is stored and metered to a xanthogen formatereactor. The xanthogen formate storage reactor is first loaded withwater into which an alkyl chloroformate is metered and agitated until anemulsion is formed and recirculated through the heat-exchanger. To thisrecirculation stream, a metered amount of xanthate solution is fed andthe reaction temperature is adjusted according to the alkyl groupsinvolved. During the reaction, the chloroformate partitions between thewater and oily phase, requiring that these two phases be meteredindependently to the heat exchanger, where the main reaction isoccurring. When the xanthate solution has all been fed to the xanthogenformate reaction, the agitation is started to complete the reaction.When the temperature stops rising or the temperature controller callsfor no more cooling, the agitation is shut off, and the phases areallowed to decant. Normally the lower phase is the oily product. This istransferred to the wash reactor where remaining alkali metal chlorideand un-reacted xanthate are removed by washing with water. The phasesare separated by decantation and the oily phase stored. The water phaseis normally discarded. The brine phase in the xanthogen formate reactor,which contains most of the alkali metal chloride produced in thereaction, is sent for treatment prior to discard. If the xanthateemployed was in an alcoholic solution, the alcohol is recovered bydistillation.

Xanthate is prepared using known methods. For alcohols which containmore than 2 carbon atoms, the xanthate reaction is performed normally inan inert organic media, such as toluene, benzene, hexane etc. Normally,the xanthate is made employing a 10 to 30% excess of alcohol and a 3 to5% excess of caustic soda referred to carbon disulfide. This ensuresthat no unreacted CS₂ remains in the xanthate which could be deleteriousif contained in the final xanthogen formate. The alcohol is added to theinert media and powdered, pellet or flake caustic added to the reactorto form the alcoholate. This reaction is highly exothermic and willrequire refluxing and strong cooling. The CS₂ is added slowly to thealcoholate maintaining the temperature at a maximum level that dependson the alkyl group involved; usually below 30° to 40° C. After thexanthate reaction is complete, the xanthate in suspension in the inertmedia is dissolved in water and the solution decanted and transferred tothe holding tank that feeds the xanthogen formate reactor.

In the case of sodium ethyl xanthate the reaction is carried out with a100 to 200% excess alcohol, and enough water to maintain the xanthate insolution. With this procedure 50% caustic may be employed, which isconsiderably cheaper than solid caustic soda.

The xanthogen formate reaction is carried out in the xanthogen formatereactor, which is first loaded with enough water so that the total wateremployed in the xanthate solution plus the amount added to the xanthogenformate reactor is capable of dissolving all the sodium chloride formedin the chloroformate reaction. To the water added to the reactor, orsimultaneously while adding the water through the recirculation pump, ameasured amount of alkyl chloroformate is added to the reactor andagitated to form an emulsion. Normally the chloroformate employed is 95to 97% molar of the theoretical amount required to react all thexanthate that will be fed to the reactor. As the yield of xanthate is ofthe order of 88 to 94% the chloroformate employed in terms of the molesof CS₂ will be 85% to 90% of that theoretically required.

At the start of the reaction, pumps No. 2 and 4 are operated torecirculate chloroformate emulsion. The xanthate solution pump (1) isstarted at about 50% of the volumetric flow of the circulating loop. Thetemperature is controlled as a function of the alkyl groups involved,and the amount of xanthic anhydride desired in the final product. After2/3 of the xanthate solution is consumed, the flow is reduced to 1/3 ofthe flow of pump No. 2 and pump No. 3 is started up to circulate theorganic phase from the bottom of the reactor. After all the xanthate istransferred, the xanthogen formate agitator is started up, whilemaintaining recirculation, to complete the reaction. The reaction iscomplete when no more cooling water is required to maintain the reactortemperature. Normally agitation is maintained for 30 to 45 minutes aftercompletion of the reaction.

After stopping agitation, the oily phase is decanted and transferred tothe wash reactor. In the case of diethyl xanthogen formate, the ethylmonothiocarbonate formed by the reaction of Equation 5 can be recoveredby adding more ethyl chloroformate to form diethoxy carbonyl sulfide(III) according to Equation 6. This is an oily product and is recoveredby decantation and washing.

The amount of wash water employed in cleaning up the main product isadjusted to remove all the alkali metal chloride, leaving 3 to 6%unreacted alcohol in the product, which helps disperse the collectorwhen added to the flotation circuit.

The main brine is distilled only in the case of the diethyl xanthogenformate, when considerable excess alcohol is employed in the manufactureof the xanthate. The recovered alcohol is returned to the xanthatereactor.

EXAMPLE 1 Production of Diethyl Xanthogen Formate

A batch of ethyl xanthogen ethyl formate was produced by preparing asodium ethyl xanthate employing 3.5 moles of ethyl alcohol which wasreacted with 1.05 moles of a 48% solution of NaOH. The reaction was fastand reached a temperature of 60° C. The alcoholate formed was cooled to22° C. and 1.0 moles of CS₂ were added at such a rate that the reactortemperature was maintained between 25° and 35° C. by the internalcooling to assure that the temperature did not drop below 25° C.,(otherwise xanthate can come out of solution).

The second reactor was loaded with 0.93 moles of ethyl chloroformate andan equal volume of water. The reactor was operated as described above,maintaining the heat exchanger outlet temperature at about 25° C. whichkept the reactor temperature below 56° C.

The above procedure was repeated without employing recirculation,agitating the reactor and controlling the bulk temperature to 56° C.using a cooling jacket. The yields on raw materials and the finalproduct composition were:

    ______________________________________                                        Raw Material       Agitated Circulating                                       Consumption.       Reactor  Reactor                                           ______________________________________                                        Kg/Kg of final product                                                        Ethyl chloroformate                                                                              0.656    0.615                                             Carbon disulfide   0.479    0.448                                             Caustic soda (48%) 0.529    0.495                                             Final product composition                                                     (G.C.)                                                                        Compound (I)       42.4     66.1                                              Compound (II)      25.0     19.2                                              Compound (III)     17.2     12.3                                              Compound (IV)      2.5      1.2                                               Volatiles          10.1     1.0                                               ______________________________________                                    

A second improvement to this process, which reduces chloroformateconsumption, is to control excess alkalinity by neutralizing thexanthate with an inorganic acid. This inhibits hydrolysis of thechloroformate by reducing the reactions of Equation (6) and (7). Thisoperation is not essential when a great excess of alcohol is employed inthe manufacture of the xanthate, as the excess of alcohol neutralizesexcess alkalinity.

EXAMPLE 2 Use of Acid Neutralization

A sample of sodium ethyl xanthate was made using 1.3 moles of ethylalcohol, 1.07 moles of NaOH and 1.0 moles of CS₂. Half the sample wasneutralized with sulfuric acid. Xanthogen formates prepared by thestandard method used in the previous example had the following yieldsand product analyses:

    ______________________________________                                        Raw Material                                                                  Consumption      Un-neutralized                                                                            Neutralized                                      ______________________________________                                        Kg/Kg of final product                                                        Ethyl chloroformate                                                                            0.672       0.612                                            Carbon disulfide 0.472       0.445                                            Caustic soda     0.551       0.529                                            Final Product Composition                                                     (G.C.)                                                                        Compound (I)     56.37       59.66                                            Compound (II)    20.80       16.27                                            Compound (III)   17.00       14.41                                            Compound (IV)    0.40        0.50                                             Volatiles        5.40        9.00                                             ______________________________________                                    

A third improvement in the process is to dissolve xanthic anhydride inthe chloroformate prior to the reaction with the xanthate and thus shiftthe equilibrium of Equation 5 to favor xanthogen formate and suppressthe formation of more xanthic anhydride, thus increasing yields onchloroformate.

EXAMPLE 3 Addition of Xanthic Anhydride to the Xanthogen FormateReaction

Four samples of ethyl xanthate were made using 2.5 moles of CS₂. Thesesamples were reacted with 0.95 moles of ethyl chloroformate containing0, 6, 12 and 18% molar of diethyl xanthic anhydride. Analysis of thefinal product gave the following composition by Gas Chromatography:

    ______________________________________                                        % Xanthic anhydride                                                                         % Each compound in final product                                in Chloroformate                                                                            (I)    (II)    (III)                                                                              (IV)  Volatiles                             ______________________________________                                         0%           66.8   15.2    13.2 1.2   3.6                                    6%           64.4   20.5    11.7 1.1   2.5                                   12%           59.7   24.2    11.7 0.9   3.5                                   18%           60.1   27.2    10.2 0.8   1.7                                   ______________________________________                                    

Normalized for the initial xanthic anhydride in the ECF the assays were:

    ______________________________________                                         0%     66.8       15.2   13.2     1.2 3.6                                     6%     68.5       15.4   12.4     1.2 2.5                                    12%     67.8       13.9   13.3     1.0 4.0                                    18%     73.3       11.2   12.4     1.0 2.1                                    ______________________________________                                    

EXAMPLE 4 Effect of Composition on Metallurgical Activity

A porphyry mineral, low in clays and with normal oxide content, easilyfloatable, whose total copper content was 1.48% with 0.013% molybdenum,was floated at 19° C. and at a pH of 4.0 for 7 minutes with the standardcollector composition in use at the mine, which corresponded to 60%collector, 30% regular gasoline and 10% MIBC (methyl isobutyl carbinol).Feed to the flotation cells was:

    ______________________________________                                        Collector Mix    80     grams/ton of mineral                                  Frother, Dowfroth 1012                                                                         40     grams/ton of mineral                                  Fuel Oil         20     grams/ton of mineral                                  Sulfuric acid    2000   grams/ton of mineral                                  ______________________________________                                    

The collector composition was:

    ______________________________________                                                     Collector Composition %                                          Test           1      2      3    4    5    6                                 ______________________________________                                        Diethyl xanthogen formate                                                                    70.1   100    51.2 51.2 60   --                                Isobutyl xanthogen ethyl                                                                     --     --     13.7 13.7 --   --                                formate                                                                       Diethyl xanthic anhydride                                                                    12.0   --     15.0 --   --   --                                Diethyl xanthic emulsified                                                                   --     --     --   15.0 --   --                                Diethoxy carbonyl sulfide                                                                     8.4   --      8.9  8.9 40   100                               Inerts          9.1   --     11.2 11.2 --   --                                ______________________________________                                    

The product used in trial #1 was the standard product employed at themine. Flotation results were as follows:

    ______________________________________                                        Concentrate  Tails                                                                  WT      %      %    WT   %    %     %     %                             Test  gm      CuT    MoT  gm   CuT  MoT   CuT   MoT                           ______________________________________                                        1     101.0   12.77  0.106                                                                              890.0                                                                              0.21 0.0024                                                                              87.3  83                            2     87.3    14.89  0.122                                                                              914.0                                                                              0.20 0.0025                                                                              87.7  82                            3     89.6    14.36  0.117                                                                              908.0                                                                              0.21 0.0027                                                                              87.1  81                            4     87.0    14.76  0.140                                                                              909.0                                                                              0.21 0.0008                                                                              87.1  94                            5     90.0    14.37  0.135                                                                              913.0                                                                              0.20 0.0010                                                                              87.6  93                            6     79.0    14.04  0.152                                                                              944.0                                                                              0.39 0.0013                                                                              75.1  91                            ______________________________________                                    

As can be seen from these results, comparing runs 1, 2 and 3 with 4, 5and 6, an increase in diethoxy carbonyl sulfide (compound III) oremulsifying the diethyl xanthic anhydride (compound II) markedlyimproves molybdenum recovery. In test 6, where the collector consistedof pure compound III, molybdenum recovery is comparably improved, butcopper is floated weakly. This indicates that molybdenum recovery can besignificantly improved by combining a xanthogen formate with anincreased amount of compound III or a soluble form of dialkyl xanthicanhydride (compound II).

EXAMPLE 5 Effect of Composition on Slime-containing Minerals

In this series of laboratory flotations, a high clay content ore fromthe same mine as in Example 4 was employed. The ore processed had copperheads of 1.45%, of which 0.36% was non-sulfide, and a molybdenum contentof 0.015%. The flotation procedure and other reagents employed were thesame as in Example 4; the standard reagent employed in test 1 wasidentical to that in the previous example. The composition of thecollectors employed were:

    ______________________________________                                                          % Composition                                               Test                1        2      3                                         ______________________________________                                        Ethyl xanthogen formate                                                                           70.5     49.9   47.3                                      Isobutyl xanthogen formate                                                                        --       13.4   12.8                                      Diethyl xanthic anhydride                                                                         12.0     11.0   10.4                                      Diethoxy carbonyl sulfide                                                                          8.4     15.0   20.0                                      Others               9.1     10.7    9.5                                      Flotation Results                                                             Total Copper recovery %                                                                           85.3     86.8   87.5                                      Total Molybdenum Recovery %                                                                       76       --     83                                        ______________________________________                                    

In this series of tests one can see that increasing the content ofcompound III when floating a high slime containing ore improves bothcopper and molybdenum recovery.

EXAMPLE 6 Control of Collector Crystallization Temperature

In most copper mines of the world, winter ambient temperatures are verylow, and, more important, flotation water temperature frequently is lessthan 10° C. during winter months. Thus, collector freezing point is animportant factor. More important is that, in the case of xanthogenformate collector, dialkyl xanthic anhydrides, where the alkyl group ismethyl, ethyl or propyl, are solids with a melting point around 50° C.,in solution in base xanthogen formate. As was shown in test 4 of Example4, maintaining the xanthic anhydride in solution significantly improvesmolybdenum recovery. In the case of diethyl xanthic anhydride, thetemperatures at which crystals of xanthic anhydride appear, as afunction of composition, are:

    ______________________________________                                                   Composition %                                                      ______________________________________                                        Ethyl xanthogen ethyl                                                                      100       70.5    62     52                                      formate                                                                       Diethyl xanthic                                                                            --        12.0    20     30                                      anhydride                                                                     Diethoxy carbonyl                                                                          --         8.4     8      8                                      sulfide                                                                       Inerts       --         9.1    10     10                                      Crystallization                                                                            -36.5° C.                                                                        -7° C.                                                                         +4° C.                                                                        +14° C.                          temperature                                                                   ______________________________________                                    

It has been found that, without a negative effect on flotation, thexanthic anhydride can be maintained in solution if methyl alcohol isadded to the alcohol employed in making the original ethyl alcohol anthe reaction conditions are adjusted to produce a xanthogen formatewhich contains 30% diethyl xanthic anhydride, the crystallization pointis reduced from +14° C. to 0° C. Up to 10% molar methanol may be addedwithout adversely affecting flotation, but the exact compound that isacting as a solubilizer has not been determined.

Various changes in the details, steps, materials and arrangements ofparts, which has been herein described and illustrated to explain thenature of the invention, may be made by those skilled in the art withinthe principle and scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An improved floation collector consistingessentially of a mixture of:(I) alkyl xanthogen formate of the generalformula ##STR10## in which R and R' are selected from the groupconsisting of alkyl groups with 1 to 6 carbon atoms, straight chained orbranched, pure or in mixtures; said formate being prepared by reactionof an alkaline alkyl xanthate and an alkyl chloroformate, and thefollowing co-products: ##STR11## wherein R and R' are defined as setforth above; and the amount of compound II in said mixture is within therange of 12 to 40%.